1. Field of the Invention
This invention relates, in general, to aircraft flight instrumentation and, more particularly, to an artificial horizon raster generator in which a two color display is specified entirely by the horizon line or transition line parameters. More specifically, the invention relates to a dedicated artificial horizon raster generator which requires no memory and removes the horizon line computation burden from the on board host processor.
2. Description of the Prior Art
Two basic methods have been used by prior art artificial horizon raster systems. The most basic method is to represent every screen pixel with a memory element. While this bit per pixel method is very flexible, the large amount of memory required is costly and requires extensive host processor computations. A typical two color display consisting of 256 lines with 256 pixels per line requires 65,536 memory elements.
By making use of the artificial horizon display's simplicity, a second prior art method uses character blocking to reduce memory requirements. The two color display is divided into many blocks, each consisting of many screen pixels. Each block is then assigned a character to define the colors of the individual pixels. Typically, 90% of the display can be produced using only two characters, one character representing sky shading and the other character representing ground shading. The remaining 10% of the display, which comprises the transition region or horizon boundary line between sky shading and ground shading, may be defined using a few more characters. A typical two color horizon display consisting of 256 lines with 256 pixels per line, divided into four by four pixel blocks, can be defined with 128 different characters assigned to the 4,096 blocks. Such an implementation requires 30,720 memory elements. While this character blocking method reduces the amount of memory required, the host processor's computation burden is still extensive and the amount of memory used remains considerable.
A technique for further reducing the host processor's computation burden is discussed in U.S. Pat. No. 4,149,148 entitled "Aircraft Flight Instrument Display System", invented by Miller et al. and assigned to the assignee of the present invention. As taught by Miller, the horizon display may be reduced to a straight line which separates the two color areas. The entire display may then be specified simply by specifying the transition line parameters, that is, the slope of the horizon line, the starting color, and the horizontal and vertical coordinates of the point at which the raster scan will first encounter or intersect the horizon boundary line. The entire display is then generated by computing each transition point intersected by each raster scan line and storing these points in memory. Also stored in memory is the video shading information representative of the appropriate sky or ground shading corresponding to each raster line. Thus the sky-ground shading is provided by addressing memory in synchronism with the raster scan, and changing the shading from sky to ground or vice versa in accordance with the information stored in the memory. It is noted that this method requires the host processor to compute each transition point intersected by the raster scan line, thus placing a burden on the host processor. Second, the transition points so computed must be stored in memory for later use. These two requirements are considered undesirable since the host processor is usually responsible for controlling a plurality of flight instruments. Assigning the processor the additional task of controlling the artificial horizon raster generator necessarily results in speed retarding interrupts and an increased memory budget. These disadvantages become even greater when one considers the impact of increasing the display resolution. For example, a change from a display of 256 lines containing 256 pixels per line to a display of 512 lines containing 512 pixels per line would double the number of host processor computations and double the amount of memory needed to store the transition points.